1. Field of the Invention
The present invention relates to an optical apparatus that has a plurality of lenses and is used for digital still cameras. In the optical apparatus, by providing additionally a kind of optical element made of glass, plastic or the like that has a higher refractive index than that of air and by inserting said element between the rearmost element of lens and the image sensor the back focal length of the lens to with shorter back focal length can be extended without changing its focal length itself.
2. Background Art
A lens turret is popularly used to obtain multiple focal lengths in a camera. However, in the case of digital still cameras (hereinafter referred to as DSC) or especially in the case of mobile phone DSC, lens modules are required to be extremely small due to the limited space for the lens module. Conventional lens turrets usually have two or three lenses of different focal lengths, and the lenses are fixed on a disk surrounding an axis, and the disk is rotatable around the axis so that one of the lenses can be set at the picture taking position. In the conventional lens turrets, the lenses are simply fixed on the disk, so the heights of lenses are all different. The lens with longer focal length is much higher than the lens with shorter focal length. FIG. 1 shows how different the heights of two lenses are as a common example. Moreover, if the lenses are positioned closely each other surrounding the axis, the lens barrel of longer focal length lens (hereinafter referred to as a telephoto lens) cuts in and disturb the light path of the shorter focal length lens (hereinafter referred to as a wide-angle lens) as shown by FIG. 2, because both of the lenses are placed at the same level on the disk. Consequently, there is a limit for shortening the distance between the lens positions, and that makes it difficult for the total size of lens turret to be compact. If the lenses are so positioned as not to disturb their light paths with each other, they should be positioned as shown in FIG. 3, and the distance between the two lenses should be much longer than the distance of the case shown in FIG. 2.
It is therefore an object of this invention to provide a lens turret that is much smaller area-wise, and much lower and flat height-wise.
A primary object of the present invention is to provide a lens turret that is extremely compact and flat in size and suitable for digital still cameras and mobile phone digital cameras comprising a lens turret rotatable around an axis and having plurality of lenses mounted thereon, a driving mechanism to rotate the lens turret so that one of the lenses can be set at the picture taking position, and a means to adjust and extend the back focal length of the wide-angle lens shaped like a column or square column and made of glass, plastic or the like that has higher refraction index than that of air.
Lenses are designed having a back focal length, which is the distance between the rearmost lens element and the focal plane of image sensor. This distance is fixed when the design was finalized. Usually when designing lenses, the designer considers the space between the rearmost element and the image sensor being filled by air, and uses the refractive index of air for his design calculation. It is a well-known fact that if an additional optical element that has a higher refractive index than that of air is inserted between the rearmost element and the image sensor, the back focal length of the lens is extended to some extent, and yet the focal length itself is not changed at all.
By making use of this phenomenon of back focal length extension, and by inserting an additional optical element between the lens and image sensor, the lens can be placed farther apart from the image sensor than its original position.
By using an additional element made of a material with a higher refractive index than the refractive index of air being placed between the lens and the image sensor, the present invention enables the lens turret to have the wide-angle lens placed higher in position than its original position so that the wide-angle lens can be placed almost at the same level as the telephoto lens, and when they are built in a DSC, the telephoto lens does not protrude and at the same time the distance between the both lenses can be closer preventing the telephoto lens barrel from disturbing the light path of the wide-angle lens so that the total size of the lens turret can be much more compact.